The present invention relates to a method of ion imaging, a method of mass spectrometry and a mass spectrometer.
Biological tissue sections for ion imaging experiments may take several hours to prepare often with a large degree of variability in matrix deposition thickness and crystal conformation. As such, the optimum parameters for generating analyte ion signals from biological tissues (or other surfaces) can vary significantly from sample to sample. Matrix Assisted Laser Desorption Ionisation (“MALDI”) is a destructive ionisation process and it is therefore important for an operator to know the best parameters to use for each sample loaded into the instrument source. Presently, the optimum parameters are found by trial and error. If non-optimum tuning parameters are used then the user not only wastes the sample but the time involved in preparing the sample and acquiring the data is also wasted.
Important tuning parameters in MALDI ionisation include the number of laser shots per pixel.
If the system is set to acquire too many shots per pixel then the sample/matrix will burn through too quickly and a large proportion of laser shots will not contribute to the analyte signal of interest and will reduce the signal to noise and increase the analysis time.
Laser energy per shot is also crucial with the optimum usually being within a narrow range of values and is heavily dependent upon the sample. The optimum value is also related to the number of laser shots parameter for each pixel. As such, tuning parameters are often non-orthogonal thereby compounding the problem.
US 2007/0141719 (Bui) discloses a method for reducing scan times in mass spectral tissue imaging studies.
US 2006/0186332 (Haase) discloses a laser system for ionisation of a sample using MALDI techniques. The characteristics of the laser beam can be altered by mechanically adjusting a lens assembly or by using a beam attenuator.
US 2011/0272573 (Kostrzewa) discloses an acquisition technique for MALDI time of flight mass spectra.
It is desired to provide an improved method of ion imaging.